usr.bin/svn: convert to OBJTOP/SRCTOP idioms

[ngie]

Summary

usr.bin/svn: convert to OBJTOP/SRCTOP idioms

The former idioms used were .CURDIR/.OBJDIR-relative paths.

Using OBJTOP/SRCTOP (as discussed previously), makes bathing more
intuitive, abbreviates make output by resolving relative paths
absolute to the top of the source tree, makes portions of the tree
relocatable/forkable, etc.

Use SVN_OBJ and SVN_SRC to parameterize out OBJTOP/usr.bin/svn and
SRCTOP/usr.bin/svn, respectively.

MFC after: 1 month
Sponsored by: Dell EMC Isilon

[ngie]

Parameterize ${SRCTOP} and ${OBJTOP} relative paths to usr.bin/svn so this
directory tree can be further relocated if need be, in a much simpler way

[ngie]

Add some previously missed Makefiles

[bdrewery]

usr.bin/svn/Makefile.inc

16–23

The usr.bin/Makefile.inc is now included less often. See usr.bin/svn/lib/Makefile.inc. If SVNDIR is not set, it then sets SVNDIR, then includes this file. Thus this line will never trigger since it is blocked by !defined(SVNDIR).

cd usr.bin/svn/lib/libserf && make -V BIN_DIR

Side note, it would be nicer if Makefile.inc changes were not bundled with .OBJDIR/CURDIR:H changes.

[ngie]

Abandoning revision since I will not be working on the build system proper anymore, apart from test integration or any components that directly correlate with testing.